Is your triple play new? Comcast will install the correct modem when they install the phone service.
If you intend on purchasing your own modem, the only user provided EMTA modem that Comcast will attach to their networks is the Arris TM722g. You can buy it from a participating Best Buy with a Comcast Connection Center in it. The price is around $150.
You can use this link to see if there is a qualifying Best Buy in your area